U.S. patent application number 17/401175 was filed with the patent office on 2021-12-02 for optical waveguide circuit.
This patent application is currently assigned to FURUKAWA ELECTRIC CO., LTD.. The applicant listed for this patent is FURUKAWA ELECTRIC CO., LTD.. Invention is credited to Junichi HASEGAWA, Noritaka MATSUBARA.
Application Number | 20210373234 17/401175 |
Document ID | / |
Family ID | 1000005822258 |
Filed Date | 2021-12-02 |
United States Patent
Application |
20210373234 |
Kind Code |
A1 |
MATSUBARA; Noritaka ; et
al. |
December 2, 2021 |
OPTICAL WAVEGUIDE CIRCUIT
Abstract
An optical waveguide circuit includes: a ring waveguide; an
input connection waveguide; an output connection waveguide; and an
optical multiplexing/demultiplexing part that optically connects
the ring waveguide with the input connection waveguide, and that
optically connects the ring waveguide with the output connection
waveguide. Further, at least one of the input connection waveguide
and the output connection waveguide includes a plurality of curved
waveguides, a sum total of products of curvature signs and bending
angles of the curved waveguides and a sum total of a curvature sign
and a bending angle of the ring waveguide have a same absolute
value with signs opposite to each other, and rotation of a
polarization plane of light generated in the ring waveguide and
rotation of a polarization plane of light generated in the curved
waveguides cancel each other out.
Inventors: |
MATSUBARA; Noritaka; (Tokyo,
JP) ; HASEGAWA; Junichi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FURUKAWA ELECTRIC CO., LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
FURUKAWA ELECTRIC CO., LTD.
Tokyo
JP
|
Family ID: |
1000005822258 |
Appl. No.: |
17/401175 |
Filed: |
August 12, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2020/004433 |
Feb 5, 2020 |
|
|
|
17401175 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/29338 20130101;
G02B 6/12007 20130101; G02B 6/125 20130101 |
International
Class: |
G02B 6/12 20060101
G02B006/12; G02B 6/125 20060101 G02B006/125 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 2019 |
JP |
2019-024922 |
Claims
1. An optical waveguide circuit comprising: a ring waveguide; an
input connection waveguide; an output connection waveguide; and an
optical multiplexing/demultiplexing part that optically connects
the ring waveguide with the input connection waveguide, and that
optically connects the ring waveguide with the output connection
waveguide, wherein at least one of the input connection waveguide
and the output connection waveguide includes a plurality of curved
waveguides, a sum total of products of curvature signs and bending
angles of the curved waveguides and a sum total of a curvature sign
and a bending angle of the ring waveguide have a same absolute
value with signs opposite to each other, and rotation of a
polarization plane of light generated in the ring waveguide and
rotation of a polarization plane of light generated in the curved
waveguides cancel each other out.
2. The optical waveguide circuit according to claim 1, wherein
curvature radii of the curved waveguides and a curvature radius of
the ring waveguide are different from each other.
3. The optical waveguide circuit according to claim 1, wherein an
absolute value of the sum total of the products of the curvature
signs and the bending angles of the curved waveguides is 360
degrees.
4. The optical waveguide circuit according to claim 1, wherein each
of the curved waveguides has a curvature radius with which light
loss through the waveguides is 1.0 dB/cm or less.
5. The optical waveguide circuit according to claim 1, wherein the
optical multiplexing/demultiplexing part includes two optical
multiplexers/demultiplexers, and one end of the input connection
waveguide and one end of the output connection waveguide are
extended toward a same side face.
6. The optical waveguide circuit according to claim 1, comprising a
plurality of the ring waveguides, wherein the input connection
waveguide includes at least one branch part and a plurality of
branched paths that are branched from the at least one branch part
and optically connected to each of the ring waveguides, the
branched paths include two or more of the curved waveguides, and
sum totals of products of curvature signs and bending angles of the
curved waveguides in each of the branched paths are equivalent with
each other.
7. The optical waveguide circuit according to claim 1, comprising a
plurality of the ring waveguides, wherein the input connection
waveguide includes at least one branch part and a plurality of
branched paths that are branched from the at least one branch part
and optically connected to each of the ring waveguides, the
branched paths include two or more of the curved waveguides, and
the ring waveguides include ring waveguides having light
circulating directions different from each other.
8. The optical waveguide circuit according to claim 7, wherein sum
totals of products of curvature signs and bending angles of the
curved waveguides in each of the branched paths are different from
each other.
9. The optical waveguide circuit according to claim 1, comprising a
plurality of the ring waveguides, wherein the ring waveguides
include ring waveguides having waveguide lengths different from
each other.
10. The optical waveguide circuit according to claim 1, wherein
when TE-polarized or TM-polarized light of single polarization is
input from the input connection waveguide, a polarization mode
coupling amount of light output from the output connection
waveguide is -25 dB or less.
11. The optical waveguide circuit according to claim 1, wherein the
input connection waveguide, the ring waveguide, the output
connection waveguide, and the optical multiplexing/demultiplexing
part are formed with a core, and the core is a buried waveguide
buried in a cladding.
12. The optical waveguide circuit according to claim 11, wherein a
relative refractive-index difference of the core with respect to
the cladding is 4.5% or more.
13. The optical waveguide circuit according to claim 11, wherein
the core contains ZrO.sub.2.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of International
Application No. PCT/JP2020/004433, filed on Feb. 5, 2020 which
claims the benefit of priority of the prior Japanese Patent
Application No. 2019-024922, filed on Feb. 14, 2019, the entire
contents of which are incorporated herein by reference.
BACKGROUND
[0002] The present disclosure relates to an optical waveguide
circuit including a ring waveguide.
[0003] There is an optical waveguide circuit used for optical
communication, which includes a ring waveguide and maintains and
outputs the polarization state of input light. This type of optical
waveguide circuit is configured with a ring waveguide, an optical
multiplexer/demultiplexer, an input connection waveguide, and an
output connection waveguide. The ring waveguide is formed by an
arc-shaped curved waveguide. The light transmission intensity of
this type of optical waveguide circuit exhibits wavelength
dependency with periodic intervals (Free Spectral Range: FSR).
"FSR" is expressed as "FSR=c/nL", where "c" is the velocity of
light, "L" is the waveguide length of the ring waveguide, and "n"
is the effective refractive index of the ring waveguide. It is
possible to set FSR as desired by adjusting the ring waveguide
length. Optical filters exhibiting various transmittance properties
are used for optical communication, and a part thereof can be
implemented by applying a ring resonator.
[0004] Polarization multiplexed signals are used in the recent
optical communication technology, and the optical waveguide circuit
is required to have a property that maintains and outputs the input
polarization (referred to as a polarization maintaining property).
However, the polarization maintaining property may be lost in
actual optical waveguide circuits due to following reasons.
[0005] In an optical waveguide formed on a substrate, there is
anisotropic stress to a waveguide section. This is because a
structure formed with a cladding and a core is spread in a plan
shape in the horizontal direction of a substrate face, but the
structure in the perpendicular direction of the substrate face is
generally thinner than that of the horizontal direction so that the
stress imposed upon the waveguide for each of the directions
varies. Furthermore, in a case where the thermal expansion
coefficients of the substrate, the lower cladding, the core, and
the upper cladding configuring the waveguide vary and if a
high-temperature heat treatment of 1000.degree. C. or higher, for
example, is performed in a waveguide manufacturing step, thermal
stress is generated in the waveguide during its cooling process.
Furthermore, if the fabrication method of the upper cladding is
Flame Hydrolysis Deposition (FHD), the volume greatly changes in
the process of depositing a porous body made of fine particles and
transparent vitrification and, in some cases, stress large enough
to deform the core may be generated. Such stress is anisotropic,
which may cause a vibration plane of each polarization mode
(vibrating direction is referred to as polarization principal axis,
vibration plane is referred to as polarization plane) to be tilted
with respect to the substrate face. This is an issue for the
optical waveguide circuit that is required to have the polarization
maintaining property to maintain and output the input
polarization.
[0006] Therefore, when light of transverse electric (TE)
polarization (polarization in the parallel direction with respect
to the substrate face) is input to the optical waveguide circuit,
light of transverse magnetic (TM) polarization may be generated.
Note here that the TM-polarization means the polarization in the
perpendicular direction with respect to the substrate face. That
is, the light of TM-polarization not existed at the time of input
is generated, and the intensity of the light of TE-polarization is
decreased. This phenomenon is called polarization coupling, which
in this case can be expressed with a polarization mode coupling
amount that is acquired by dividing the TM-polarization component
intensity by the TE-polarization component intensity. In the usage
where the polarization is to be maintained, it is preferable to
have a higher polarization maintaining property. That is, it is
preferable to have a smaller polarization mode coupling amount, and
-20 dB or less is generally desired.
[0007] Japanese Patent No. 5959505 discloses a technique in which
curved sections having the curvature of reverse signs, equivalent
curvature radius, and equivalent arc angle are combined so as to
cancel out rotations of the polarization planes of light generated
in the curved sections.
SUMMARY
[0008] There is a need for providing an optical waveguide circuit
in which both high flexibility in designing FSR of the ring
resonator and low bending loss while securing a high polarization
maintaining property can be realized.
[0009] According to an embodiment, an optical waveguide circuit
includes: a ring waveguide; an input connection waveguide; an
output connection waveguide; and an optical
multiplexing/demultiplexing part that optically connects the ring
waveguide with the input connection waveguide, and that optically
connects the ring waveguide with the output connection waveguide.
Further, at least one of the input connection waveguide and the
output connection waveguide includes a plurality of curved
waveguides, a sum total of products of curvature signs and bending
angles of the curved waveguides and a sum total of a curvature sign
and a bending angle of the ring waveguide have a same absolute
value with signs opposite to each other, and rotation of a
polarization plane of light generated in the ring waveguide and
rotation of a polarization plane of light generated in the curved
waveguides cancel each other out.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic view of an optical waveguide circuit
according to a first embodiment;
[0011] FIG. 2 is a fragmentary sectional view of the optical
waveguide circuit illustrated in FIG. 1;
[0012] FIG. 3 is a chart illustrating an example of a relation in
regards to the bending radius, the bending loss, and FSR;
[0013] FIG. 4 is a schematic view of an optical waveguide circuit
according to a second embodiment;
[0014] FIG. 5 is a schematic view of an optical waveguide circuit
according to a third embodiment; and
[0015] FIG. 6 is a schematic view of an optical waveguide circuit
according to a fourth embodiment.
DETAILED DESCRIPTION
[0016] In the related art, following issues may arise when the
curved sections having the curvature of reverse signs, equivalent
curvature radius, and equivalent arc angle are combined in the
optical waveguide circuit including the ring waveguide, the optical
multiplexer/demultiplexer, the input connection waveguide, and the
output connection waveguide in order to implement a high
polarization maintaining property. That is, as the property of the
curved sections, combinations of the signs of curvature, the
curvature radii, and the arc angles are limited so that it may not
be possible to design FSR of the ring resonator freely. On the
other hand, when the property of the curved sections is determined
to have a desired value for FSR of the ring resonator, it is
necessary to provide curved sections in the input connection
waveguide and the output connection waveguide accordingly, which
may cause excessive bending loss in the curved sections.
[0017] Hereinafter, embodiments will be described with reference to
the accompanying drawings. Note that the present disclosure is not
limited by the embodiments. In the accompanying drawings, same
reference signs are applied to the same or corresponding elements
as appropriate. Furthermore, it is to be noted that the drawings
are schematically illustrated and dimensional relations, ratios,
and the like of each of the elements may be different from those of
actual ones. Furthermore, between each of the drawings, there may
be a part having a different dimensional relation and ratio with
respect to each other.
First Embodiment
[0018] FIG. 1 is a schematic view of an optical waveguide circuit
according to a first embodiment. An optical waveguide circuit 100
is configured with a planar lightwave circuit (PLC) that includes
an optical waveguide 110 configured with a core and a cladding
120.
[0019] FIG. 2 is a fragmentary sectional view of the optical
waveguide circuit 100. The cladding 120 surrounds the optical
waveguide 110, and includes a lower cladding 121 and an upper
cladding 122. The cladding 120 is formed on a silicon substrate or
a glass substrate, for example. The cladding 120 is formed with a
silica-based glass material. As illustrated, TE-polarization is the
polarization in a parallel direction with respect to the substrate
face, and TM-polarization is the polarization in a perpendicular
direction with respect to the substrate face.
[0020] The optical waveguide 110 is formed with a silica-based
glass material having a higher refractive index than that of the
cladding 120. As such a silica-based glass material having a high
refractive index, it is possible to use silica glass containing
germania (GeO.sub.2) or zirconia (ZrO.sub.2) as dopant for
increasing the refractive index, for example. Especially, with the
so-called SiO.sub.2--ZrO.sub.2 based material that is silica glass
containing zirconia, a relative refractive-index difference of the
optical waveguide 110 with respect to the cladding 120 can be
increased to be 4.5% or more, for example. Therefore, it is
preferable when reducing the size of the optical waveguide circuit
100. The optical waveguide 110 configured with the core is a buried
waveguide buried in the cladding 120.
[0021] In the embodiment, it is assumed that the relative
refractive-index difference of the optical waveguide 110 with
respect to the cladding 120 is 4.5%.
[0022] Returning to FIG. 1, the optical waveguide 110 includes an
input connection waveguide 111, an output connection waveguide 112,
a ring waveguide 113, and an optical multiplexer/demultiplexer 114
as an optical multiplexing/demultiplexing part.
[0023] The ring waveguide 113 is in a circular shape having a
specific radius. The input connection waveguide 111 includes a
plurality of curved waveguides. Specifically, the input connection
waveguide 111 is configured with a straight waveguide 111a, a
curved waveguide 111b, a straight waveguide 111c, a curved
waveguide 111d, and a straight waveguide 111e. One end of the input
connection waveguide 111 is extended toward the side face of the
optical waveguide circuit 100 on the left side of the drawing.
[0024] The output connection waveguide 112 is configured with a
straight waveguide. One end of the output connection waveguide 112
is extended toward the side face of the optical waveguide circuit
100 on the right side of the drawing.
[0025] The optical multiplexer/demultiplexer 114 optically connects
the ring waveguide 113 with the input connection waveguide 111 and
the ring waveguide 113 with the output connection waveguide 112.
The optical multiplexer/demultiplexer 114 is a 2.times.2 type with
two ports on the input side and two ports on the output side, and
examples thereof may be a directional coupling type and a multimode
interference (MMI) type.
[0026] The curvature radii of the curved waveguide 111b and the
curved waveguide 111d are different from the curvature radius of
the ring waveguide 113. For example, the curvature radii of the
curved waveguide 111b and the curved waveguide 111d are 250 .mu.m.
The curvature radius of the ring waveguide 113 is 206 .mu.m.
[0027] Furthermore, the light guiding direction of the curved
waveguide 111b and the curved waveguide 111d is counterclockwise as
indicated by an arrow, and the curvature sign is negative. A
bending angle .theta.11 of the curved waveguide 111b is 270
degrees. A bending angle .theta.12 of the curved waveguide 111d is
90 degrees. Therefore, the sum total of the products of the
curvature signs and the bending angles of the curved waveguide 111b
and the curved waveguide 111d is -360 degrees.
[0028] Meanwhile, the light guiding direction of the ring waveguide
113 is clockwise as indicated by an arrow, and the curvature sign
is positive. The bending angle of the ring waveguide 113 is 360
degrees. Therefore, the sum total of the product of the curvature
sign and the bending angle of the ring waveguide 113 is 360
degrees. It is to be noted that the sum total of the product of the
curvature sign and the bending angle of the ring waveguide 113 is
not the sum total from the optical multiplexing/demultiplexing part
on the input side to the optical multiplexing/demultiplexing part
on the output side but the sum total of the product of the
curvature sign and the bending angle of the entire (a circle of)
ring waveguide 113.
[0029] As described, the sum total of the products of the curvature
signs and the bending angles of the curved waveguide 111b and the
curved waveguide 111d and the sum total of the product of the
curvature sign and the bending angle of the ring waveguide 113 have
the same absolute value with opposite signs. Thereby, rotation of
the polarization plane of light generated in the ring waveguide 113
and rotation of the polarization plane of light generated in the
curved waveguide 111b and the curved waveguide 111d cancel each
other out.
[0030] Therefore, assuming that light L11 input from the input
connection waveguide 111 is TE-polarized light of single
polarization, light L12 output from the output connection waveguide
112 is also substantially the TE-polarized light. Therefore, a high
polarization maintaining property is secured. Similarly, in a case
where the light L11 is TM-polarized light, the light L12 is also
substantially the TM-polarized light so that a high polarization
maintaining property is secured.
[0031] FIG. 3 is a chart illustrating an example of the relation in
regards to the bending radius (Radius), the bending loss (Bending
Loss), and FSR. In the optical waveguide circuit 100, the curvature
radii of the curved waveguide 111b and the curved waveguide 111d
are different from the curvature radius of the ring waveguide 113,
so that each of those can be set as an optimal value. For example,
by setting the curvature radius of the ring waveguide 113 as 206
.mu.m, FSR can be designed to be 150 GHz. Furthermore, by setting
the curvature radii of the curved waveguide 111b and the curved
waveguide 111d as 250 .mu.m, it is possible to decrease light loss
(mainly bending loss) through the waveguides to be about 1 dB/cm or
less and to reduce the size of the optical waveguide circuit
100.
[0032] For example, FIG. 3 is a chart illustrating an example of
the relation in regards to the bending radius (Radius), the bending
loss (Bending Loss), and FSR.
[0033] In a case of a technique in which curved sections having the
curvatures of reverse signs, equivalent curvature radii, and
equivalent arc angles are combined as disclosed in Japanese Patent
No. 5959505, when the curvature radii of the curved waveguide 111b
and the curved waveguide 111d are set as 250 .mu.m, it is necessary
to set the curvature radius of the ring waveguide 113 as 250 .mu.m.
In this case, while the bending loss can be set as 1 dB/cm, FSR is
fixed as 124 GHz. Meanwhile, when the curvature radius of the ring
waveguide 113 is set as 206 .mu.m so as to set FSR as 150 GHz, it
is necessary to set the curvature radii of the curved waveguide
111b and the curved waveguide 111d as 206 .mu.m as well. In this
case, the light loss is increased than the case of setting the
curvature radii of the curved waveguide 111b and the curved
waveguide 111d as 250 .mu.m, and the bending loss exceeds 1
dB/cm.
[0034] As described above, with the optical waveguide circuit 100,
it is possible to implement both high flexibility in designing FSR
of the ring waveguide 113 and low bending loss while securing a
high polarization maintaining property. Note that the curvature
radii and the bending angles mentioned above are examples, and it
is possible to set the values thereof freely in accordance with the
required bending loss and FSR.
[0035] The optical waveguide circuit 100 can be manufactured
through following steps, for example. First, using the FHD method,
fine particles of silica-based glass are deposited on a substrate,
heat is applied thereto to make the glass fine particles to
transparent glass so as to form the lower cladding 121. Then, a
silica-based glass fine particle layer to be a core is deposited on
the lower cladding 121 by sputtering. At this time, ZrO.sub.2 is
added to SiO.sub.2 to make the refractive index of the silica-based
glass fine particle layer to be the core higher than that of the
cladding such that the relative refractive-index difference between
the core and the cladding becomes 4.5.degree.. Then, an etching
mask made with resist is formed by photolithography using a
photomask having a circuit pattern of the optical waveguide 110 of
the optical waveguide circuit 100. Subsequently, by using an
etching gas such as a fluorine-based gas, dry etching is performed
on the silica-based glass fine particle layer not covered by the
etching mask. Finally, using the FHD method, fine particles of
silica-based glass are deposited, heat is applied thereto to make
the glass fine particles to transparent glass so as to form the
upper cladding 122 that configures the upper part of the cladding
120. In the manner described above, the optical waveguide circuit
100 can be manufactured.
[0036] As a first example, an optical waveguide circuit having the
configuration of the optical waveguide circuit 100 was manufactured
by the manufacturing method described above. The property of the
manufactured optical waveguide circuit was measured, and FSR was
150 GHz. Furthermore, when TE-polarized light was input from an
input connection waveguide and the polarization mode coupling
amount was measured, a preferable value of -25 dB or less was
acquired. Moreover, when a curved waveguide part of the input
connection waveguide was diced and the light loss thereof was
measured, acquired was a preferable value of 0.95 dB/cm or
less.
[0037] As a first comparative example, an optical waveguide circuit
having the same configuration as that of the first example was
manufactured except that the curvature radii of the two curved
waveguides of the input connection waveguide and the ring waveguide
were all set as 250 .mu.m. While the polarization mode coupling
amount was a preferable value of -25 dB or less, acquired FSR was
not 150 GHz as desired but 124 GHz.
[0038] Furthermore, as a second comparative example 2, an optical
waveguide circuit having the same configuration as that of the
first example was manufactured except that a single S-shaped curved
waveguide was provided between two straight waveguides for each of
the input connection waveguide and the output connection waveguide
and that the curvature radii of the S-like curved waveguides and
the ring waveguide were all set as 250 .mu.m. In this case,
rotation of the polarization plane of light generated in each of
the input connection waveguide and the output connection waveguide
is canceled within the S-like curved waveguide. However, when the
property of the optical waveguide circuit was measured, acquired
was the polarization mode coupling amount of -13 dB, which is not a
proper value for practical use.
Second Embodiment
[0039] FIG. 4 is a schematic view of an optical waveguide circuit
according to a second embodiment. An optical waveguide circuit 200
is configured with a PLC that includes an optical waveguide 210
configured with a core and a cladding 220.
[0040] The sectional structure of the optical waveguide circuit
200, the materials of the optical waveguide 210 and the cladding
220, and the relative refractive-index difference thereof may be
the same as those of the corresponding elements of the optical
waveguide circuit 100 according to the first embodiment.
[0041] The optical waveguide 210 includes an input connection
waveguide 211, an output connection waveguide 212, a ring waveguide
213, and two optical multiplexers/demultiplexers 214a and 214b
configuring optical multiplexing/demultiplexing parts.
[0042] The ring waveguide 213 is in a circular shape having a
specific radius. The input connection waveguide 211 is configured
with a straight waveguide. One end of the input connection
waveguide 211 is extended toward the side face of the optical
waveguide circuit 200 on the left side of the drawing.
[0043] The output connection waveguide 212 includes a plurality of
curved waveguides. Specifically, the output connection waveguide
212 is configured with a straight waveguide 212a, a curved
waveguide 212b, a straight waveguide 212c, a curved waveguide 212d,
and a straight waveguide 212e. One end of the output connection
waveguide 212 is extended toward the side face of the optical
waveguide circuit 200 on the left side of the drawing. That is, one
end of the input connection waveguide 211 and one end of the output
connection waveguide 212 are extended toward the same side
face.
[0044] The optical multiplexer/demultiplexer 214a optically
connects the ring waveguide 213 with the input connection waveguide
211. The optical multiplexer/demultiplexer 214b optically connects
the ring waveguide 213 with the output connection waveguide 212.
The optical multiplexers/demultiplexers 214a and 214b are a
2.times.2 type that is a directional coupling type or an MMI type,
for example.
[0045] The curvature radii of the curved waveguide 212b and the
curved waveguide 212d are different from the curvature radius of
the ring waveguide 213. For example, the curvature radii of the
curved waveguide 212b and the curved waveguide 212d are 250 .mu.m.
The curvature radius of the ring waveguide 213 is 206 .mu.m.
[0046] Furthermore, the light guiding direction of the curved
waveguide 212b and the curved waveguide 212d is counterclockwise as
indicated by an arrow, and the curvature sign is negative. The
bending angle of the curved waveguide 212b is 90 degrees. The
bending angle of the curved waveguide 212d is 270 degrees.
Therefore, the sum total of the products of the curvature signs and
the bending angles of the curved waveguide 212b and the curved
waveguide 212d is -360 degrees.
[0047] Meanwhile, the light guiding direction of the ring waveguide
213 is clockwise as indicated by an arrow, and the curvature sign
is positive. The bending angle of the ring waveguide 213 is 360
degrees. Therefore, the sum total of the product of the curvature
sign and the bending angle of the ring waveguide 213 is 360
degrees.
[0048] As described, the sum total of the products of the curvature
signs and the bending angles of the curved waveguide 212b and the
curved waveguide 212d and the sum total of the product of the
curvature sign and the bending angle of the ring waveguide 213 have
the same absolute value with opposite signs. Thereby, rotation of
the polarization plane of light generated in the ring waveguide 213
and rotation of the polarization plane of light generated in the
curved waveguide 212b and the curved waveguide 212d cancel each
other out. As a result, regardless of whether input light L21 is in
TE-polarization or TM-polarization, light L22 is output while
securing a high polarization maintaining property.
[0049] In the optical waveguide circuit 200, the curvature radii of
the curved waveguide 212b and the curved waveguide 212d are
different from the curvature radius of the ring waveguide 213, so
that each of those can be set as an optimal value. For example, by
setting the curvature radius of the ring waveguide 213 as 206
.mu.m, FSR can be designed to be 150 GHz. Furthermore, by setting
the curvature radii of the curved waveguide 212b and the curved
waveguide 212d as 250 .mu.m, it is possible to decrease light loss
(mainly bending loss) through the waveguides to be about 1 dB/cm or
less and to reduce the size of the optical waveguide circuit
200.
[0050] As described above, with the optical waveguide circuit 200,
it is possible to implement both high flexibility in designing FSR
of the ring waveguide 213 and low bending loss while securing a
high polarization maintaining property. Note that the curvature
radii and the bending angles mentioned above are examples, and it
is possible to set the values thereof freely in accordance with the
required bending loss and FSR. Furthermore, in the optical
waveguide circuit 200, one end of the input connection waveguide
211 and one end of the output connection waveguide 212 are extended
toward the same side face. Therefore, it is easily connected to an
optical fiber array or another optical waveguide circuit.
[0051] As a second example, an optical waveguide circuit having the
configuration of the optical waveguide circuit 200 was manufactured
by the manufacturing method described above. The property of the
manufactured optical waveguide circuit was measured, and FSR was
150 GHz. Furthermore, when TE-polarized light was input from an
input connection waveguide and the polarization mode coupling
amount was measured, a preferable value of -25 dB or less was
acquired. Moreover, when a curved waveguide part of the output
connection waveguide was diced and the light loss thereof was
measured, acquired was a preferable value of 0.95 dB/cm or
less.
Third Embodiment
[0052] FIG. 5 is a schematic view of an optical waveguide circuit
according to a third embodiment. An optical waveguide circuit 300
is configured with a PLC that includes an optical waveguide 310
configured with a core and a cladding 320.
[0053] The sectional structure of the optical waveguide circuit
300, the materials of the optical waveguide 310 and the cladding
320, and the relative refractive-index difference thereof may be
the same as those of the corresponding elements of the optical
waveguide circuit 100 according to the first embodiment.
[0054] The optical waveguide 310 includes: an input connection
waveguide 311; two output connection waveguides 312a and 312b; a
plurality of, two in the embodiment, ring waveguides 313a and 313b;
and four optical multiplexers/demultiplexers 314aa, 314ab, 314ba
and 314bb configuring the optical multiplexing/demultiplexing
parts.
[0055] The ring waveguides 313a and 313b are formed including
curved waveguides of a specific radius with a bending angle of 90
degrees, and formed in an elliptical shape including the optical
multiplexers/demultiplexers 314aa, 314ab, 314ba, and 314bb. The
waveguide lengths of the ring waveguides 313a and 313b are
equivalent with each other, and the circulating directions of light
are also the same with respect to each other as indicated by
arrows.
[0056] The input connection waveguide 311 includes a plurality of
curved waveguides and a branch part. Specifically, the input
connection waveguide 311 is configured with a straight waveguide
311a, a curved waveguide 311b, a straight waveguide 311c, a branch
part 311d, a curved waveguide 311e, a straight waveguide 311f, a
curved waveguide 311g, a curved waveguide 311h, a straight
waveguide 311i, a curved waveguide 311j, a straight waveguide 311k,
a curved waveguide 311l, a curved waveguide 311m, and a straight
waveguide 311n. One end of the input connection waveguide 311 is
extended toward the side face of the optical waveguide circuit 300
on the left side of the drawing. The branch part 311d branches the
input connection waveguide 311. The curved waveguide 311e, the
straight waveguide 311f, and the curved waveguide 311g configure a
first branched path that is branched from the branch part 311d and
optically connected to the ring waveguide 313a. The first branched
path includes the two curved waveguides 311e and 311g. The curved
waveguide 311h, the straight waveguide 311i, the curved waveguide
311j, the straight waveguide 311k, the curved waveguide 311l, the
curved waveguide 311m, and the straight waveguide 311n configure a
second branched path that is branched from the branch part 311d and
optically connected to the ring waveguide 313b. The second branched
path includes the four curved waveguides 311h, 311j, 311l, and
311m. While the light branching ratio at the branch part 311d is
1:1, for example, there is no specific limit set thereto. The
branch part 311d is a 1.times.2 type that is a directional coupling
type or an MMI type, for example.
[0057] The curved waveguide 311l and the curved waveguide 311m are
continuous, but the signs of the curvatures are different.
Therefore, those can be considered as individual curved waveguides.
In the current description, two continuous curved waveguides are
considered as individual curved waveguides if the signs of
curvatures or the curvature radii thereof are different.
[0058] The output connection waveguide 312a is configured with a
curved waveguide 312aa and a straight waveguide 312ab. The output
connection waveguide 312b is configured with a curved waveguide
312ba, a straight waveguide 312bb, a curved waveguide 312bc, a
straight waveguide 312bd, a curved waveguide 312be, and a straight
waveguide 312bf. One ends of the output connection waveguides 312a
and 312b are extended toward the side face of the optical waveguide
circuit 300 on the left side of the drawing. That is, one end of
the input connection waveguide 311 and one ends of the output
connection waveguides 312a and 312b are extended toward the same
side face.
[0059] The optical multiplexer/demultiplexer 314aa optically
connects the ring waveguide 313a and the curved waveguide 311g that
configures the first branched path on the input connection
waveguide 311. The optical multiplexer/demultiplexer 314ab
optically connects the ring waveguide 313a and the curved waveguide
312aa of the output connection waveguide 312a. The optical
multiplexer/demultiplexer 314ba optically connects the ring
waveguide 313b and the straight waveguide 311n that configures the
second branched path on the input connection waveguide 311. The
optical multiplexer/demultiplexer 314bb optically connects the ring
waveguide 313b and the curved waveguide 312ba of the output
connection waveguide 312b. The optical multiplexers/demultiplexers
314aa, 314ab, 314ba, and 314bb are a 2.times.2 type that is a
directional coupling type or an MMI type, for example.
[0060] The curvature radii of each of the curved waveguides of the
input connection waveguide 311 and the output connection waveguides
312a and 312b are different from the curvature radii of the curved
waveguides of the ring waveguides 313a and 313b. For example, the
curvature radii of each of the curved waveguides of the input
connection waveguide 311 and the output connection waveguides 312a
and 312b are 250 .mu.m. The curvature radii of the curved
waveguides of the ring waveguides 313a and 313b are 412 .mu.m.
[0061] As for the curved waveguides 311b, 311e, and 311g of the
input connection waveguide 311 to the ring waveguide 313a, the
light guiding direction is counterclockwise as indicated by an
arrow, and the curvature signs are negative. The bending angles of
each of the curved waveguides 311b, 311e, and 311g are 90 degrees.
Therefore, the sum total of the products of the curvature signs and
the bending angles of the curved waveguides 311b, 311e, and 311g is
-270 degrees.
[0062] Meanwhile, the light guiding direction of the curved
waveguide of the ring waveguide 313a is clockwise as indicated by
an arrow, and the curvature sign is positive. The bending angle of
the curved waveguide of the ring waveguide 313a is 360 degrees.
Therefore, the sum total of the product of the curvature sign and
the bending angle of the ring waveguide 313a is 360 degrees.
[0063] Furthermore, the light guiding direction of the curved
waveguide 312aa of the output connection waveguide 312a is
counterclockwise, the curvature sign is negative, and the bending
angle is 90 degrees. Therefore, the sum total of the product of the
curvature sign and the bending angle of the curved waveguide 312aa
is -90 degrees.
[0064] As a result, the sum total of the products of the curvature
signs and the bending angles of the curved waveguides 311b, 311e,
311g, and 312aa and the sum total of the product of the curvature
sign and the bending angle of the ring waveguide 313a have the same
absolute value with opposite signs. Thereby, rotation of the
polarization plane of light generated in the ring waveguide 313a
and rotation of the polarization plane of light generated in the
curved waveguides 311b, 311e, 311g, and 312aa cancel each other
out. As a result, regardless of whether input light L31 is in
TE-polarization or TM-polarization, light L32 is output while
securing a high polarization maintaining property.
[0065] Similarly, as for the curved waveguides 311b, 311h, 311j,
3111, and 311m of the input connection waveguide 311 to the ring
waveguide 313b, the light guiding directions are counterclockwise
or clockwise, and the curvature signs are negative or positive. The
bending angles of each of the curved waveguides 311b, 311h, 311j,
3111, and 311m are 90 degrees. Therefore, the sum total of the
products of the curvature signs and the bending angles of the
curved waveguides 311b, 311h, 311j, 3111, and 311m is -270
degrees.
[0066] That is, the sum totals of the curvature signs and the
bending angles of the curved waveguides in the first branched path
and the second branched path are equivalent with each other.
[0067] Meanwhile, the light guiding direction of the curved
waveguide of the ring waveguide 313b is clockwise as indicated by
an arrow, and the curvature sign is positive. The bending angle of
the curved waveguide of the ring waveguide 313b is 360 degrees.
Therefore, the sum total of the product of the curvature sign and
the bending angle of the ring waveguide 313b is 360 degrees.
[0068] Furthermore, as for the curved waveguides 312ba, 312bc, and
312be of the output connection waveguide 312b, the light guiding
directions are counterclockwise or clockwise, the curvature signs
are negative or positive, and the bending angles are 90 degrees.
Therefore, the sum total of the products of the curvature signs and
the bending angles of the curved waveguides 312ba, 312bc, and 312be
is -90 degrees.
[0069] As a result, the sum total of the products of the curvature
signs and the bending angles of the curved waveguides 311b, 311h,
311j, 3111, 311m, 312ba, 312bc, and 312be and the sum total of the
product of the curvature sign and the bending angle of the ring
waveguide 313b have the same absolute value with opposite signs.
Thereby, rotation of the polarization plane of light generated in
the ring waveguide 313b and rotation of the polarization plane of
light generated in the curved waveguides 311b, 311h, 311j, 3111,
311m, 312ba, 312bc, and 312be cancel each other out. As a result,
regardless of whether input light L31 is in TE-polarization or
TM-polarization, light L33 is output while securing a high
polarization maintaining property.
[0070] In the optical waveguide circuit 300, the curvature radii of
each of the curved waveguides of the input connection waveguide 311
and the output connection waveguides 312a and 312b are different
from the curvature radii of the ring waveguides 313a and 313b so
that each of those can be set as an optimal value. For example, by
setting the curvature radii of the ring waveguides 313a and 313b as
412 .mu.m, FSR can be designed to be 75 GHz. Furthermore, by
setting the curvature radii of each of the curved waveguides of the
input connection waveguide 311 and the output connection waveguides
312a and 312b as 250 .mu.m, it is possible to decrease light loss
through the waveguides and to reduce the size of the optical
waveguide circuit 300.
[0071] As described above, with the optical waveguide circuit 300,
it is possible to implement both high flexibility in designing FSR
and low bending loss while securing a high polarization maintaining
property. Note that the curvature radii and the bending angles
mentioned above are examples, and it is possible to set the values
thereof freely in accordance with the required bending loss and
FSR. Furthermore, in the optical waveguide circuit 300, one end of
the input connection waveguide 311 and one ends of the output
connection waveguides 312a and 312b are extended toward the same
side face. Therefore, it is easily connected to an optical fiber
array or another optical waveguide circuit.
[0072] As a third example, an optical waveguide circuit having the
configuration of the optical waveguide circuit 300 was manufactured
by the manufacturing method described above. The property of the
manufactured optical waveguide circuit was measured, and FSR of a
ring resonator configured with two ring waveguides was 75 GHz.
Furthermore, when TE-polarized light was input from an input
connection waveguide and the polarization mode coupling amount was
measured, a preferable value of -25 dB or less was acquired.
Moreover, when curved waveguide parts of the input connection
waveguide and the output connection waveguide were diced and the
light loss thereof was measured, acquired was a preferable value of
0.95 dB/cm or less.
Fourth Embodiment
[0073] FIG. 6 is a schematic view of an optical waveguide circuit
according to a fourth embodiment. An optical waveguide circuit 400
is configured with a PLC that includes an optical waveguide 410
configured with a core and a cladding 420.
[0074] The sectional structure of the optical waveguide circuit
400, the materials of the optical waveguide 410 and the cladding
420, and the relative refractive-index difference thereof may be
the same as those of the corresponding elements of the optical
waveguide circuit 100 according to the first embodiment.
[0075] The optical waveguide 410 includes: an input connection
waveguide 411; two output connection waveguides 412a and 412b; a
plurality of, two in the embodiment, ring waveguides 413a and 413b;
and four optical multiplexers/demultiplexers 414aa, 414ab, 414ba
and 414bb configuring the optical multiplexing/demultiplexing
parts.
[0076] The ring waveguides 413a and 413b are formed including
curved waveguides having a specific radius with a bending angle of
90 degrees, and formed in an elliptical shape including the optical
multiplexers/demultiplexers 414aa, 414ab, 414ba, and 414bb. The
ring waveguide 413b also includes a straight waveguide.
Furthermore, the waveguide lengths of the ring waveguides 413a and
413b are different from each other, and the circulating directions
of light are also different from each other as indicated by
arrows.
[0077] The input connection waveguide 411 includes a plurality of
curved waveguides and a branch part. Specifically, the input
connection waveguide 411 is configured with a straight waveguide
411a, a curved waveguide 411b, a curved waveguide 411c, a straight
waveguide 411d, a curved waveguide 411e, a branch part 411f, a
straight waveguide 411g, a curved waveguide 411h, a straight
waveguide 411i, a curved waveguide 411j, a straight waveguide 411k,
a curved waveguide 411l, a curved waveguide 411m, and a straight
waveguide 411n. One end of the input connection waveguide 411 is
extended toward the side face of the optical waveguide circuit 400
on the left side of the drawing. The branch part 411f branches the
input connection waveguide 411. The straight waveguide 411g, the
curved waveguide 411h, the straight waveguide 411i, and the curved
waveguide 411j configure a first branched path that is branched
from the branch part 411f and optically connected to the ring
waveguide 413a. The first branched path includes the two curved
waveguides 411h and 411j. The straight waveguide 411k, the curved
waveguide 411l, the curved waveguide 411m, and the straight
waveguide 411n configure a second branched path that is branched
from the branch part 411f and optically connected to the ring
waveguide 413b. The second branched path includes the two curved
waveguides 411l and 411m. While the light branching ratio at the
branch part 411f is 1:1, for example, there is no specific limit
set thereto. The branch part 411f is a 1.times.2 type that is a
directional coupling type or an MMI type, for example.
[0078] The output connection waveguide 412a is configured with a
curved waveguide 412aa and a straight waveguide 412ab. The output
connection waveguide 412b is configured with a curved waveguide
412ba, a straight waveguide 412bb, a curved waveguide 412bc, a
straight waveguide 412bd, a curved waveguide 412be, a straight
waveguide 412bf, a curved waveguide 412bg, a straight waveguide
412bh, a curved waveguide 412bi, a straight waveguide 412bj, a
curved waveguide 412bk, and a straight waveguide 412b1. One ends of
the output connection waveguides 412a and 412b are extended toward
the side face of the optical waveguide circuit 400 on the left side
of the drawing. That is, one end of the input connection waveguide
411 and one ends of the output connection waveguides 412a and 412b
are extended toward the same side face.
[0079] The optical multiplexer/demultiplexer 414aa optically
connects the ring waveguide 413a and the curved waveguide 411j that
configures the first branched path on the input connection
waveguide 411. The optical multiplexer/demultiplexer 414ab
optically connects the ring waveguide 413a and the curved waveguide
412aa of the output connection waveguide 412a. The optical
multiplexer/demultiplexer 414ba optically connects the ring
waveguide 413b and the straight waveguide 411n that configures the
second branched path on the input connection waveguide 411. The
optical multiplexer/demultiplexer 414bb optically connects the ring
waveguide 413b and the curved waveguide 412ba of the output
connection waveguide 412b. The optical multiplexers/demultiplexers
414aa, 414ab, 414ba, and 414bb are a 2.times.2 type that is a
directional coupling type or an MMI type, for example.
[0080] The curvature radii of each of the curved waveguides of the
input connection waveguide 411 and the output connection waveguides
412a and 412b are different from the curvature radii of the curved
waveguides of the ring waveguides 413a and 413b. For example, the
curvature radii of each of the curved waveguides of the input
connection waveguide 411 and the output connection waveguides 412a
and 412b are 250 .mu.m. The curvature radii of the curved
waveguides of the ring waveguides 413a and 413b are 412 .mu.m. The
length of the straight waveguide of the ring waveguide 413b is 1244
.mu.m.
[0081] As for the curved waveguides 411b, 411c, 411e, 411h, and
411j of the input connection waveguide 411 to the ring waveguide
413a, the light guiding directions are counterclockwise or
clockwise, the curvature signs are negative or positive, and the
bending angles are 60 degrees or 90 degrees. The sum total of the
products of the curvature signs and the bending angles of the
curved waveguides 411b, 411c, 411e, 411h, and 411j is -270
degrees.
[0082] Meanwhile, the light guiding direction of the curved
waveguide of the ring waveguide 413a is clockwise as indicated by
an arrow, and the curvature sign is positive. The bending angle of
the curved waveguide of the ring waveguide 413a is 360 degrees.
Therefore, the sum total of the product of the curvature sign and
the bending angle of the ring waveguide 413a is 360 degrees.
[0083] Furthermore, the light guiding direction of the curved
waveguide 412aa of the output connection waveguide 412a is
counterclockwise, the curvature sign is negative, and the bending
angle is 90 degrees. Therefore, the sum total of the product of the
curvature sign and the bending angle of the curved waveguide 412aa
is -90 degrees.
[0084] As a result, the sum total of the products of the curvature
signs and the bending angles of the curved waveguides 411b, 411c,
411e, 411h, 411j and 412aa and the sum total of the product of the
curvature sign and the bending angle of the ring waveguide 413a
have the same absolute value with opposite signs. Thereby, rotation
of the polarization plane of light generated in the ring waveguide
413a and rotation of the polarization plane of light generated in
the curved waveguides 411b, 411c, 411e, 411h, 411j and 412aa cancel
each other out. As a result, regardless of whether input light L41
is in TE-polarization or TM-polarization, light L42 is output while
securing a high polarization maintaining property.
[0085] Similarly, as for the curved waveguides 411b, 411c, 411e,
4111 and 411m of the input connection waveguide 411 to the ring
waveguide 413b, the light guiding directions are counterclockwise
or clockwise, the curvature signs are negative or positive, and the
bending angles are 60 degrees or 90 degrees. The sum total of the
products of the curvature signs and the bending angles of the
curved waveguides 411b, 411c, 411e, 4111 and 411m is -90
degrees.
[0086] That is, the sum totals of the curvature signs and the
bending angles of the curved waveguides in the first branched path
and the second branched path are different from each other.
[0087] Meanwhile, the light guiding direction of the curved
waveguide of the ring waveguide 413b is counterclockwise as
indicated by an arrow, and the curvature sign is negative. The
bending angle of the curved waveguide of the ring waveguide 413b is
360 degrees. Therefore, the sum total of the product of the
curvature sign and the bending angle of the ring waveguide 413b is
-360 degrees.
[0088] Furthermore, as for the curved waveguides 412ba, 412bc,
412be, 412bg, 412bi, and 412bk of the output connection waveguide
412b, the light guiding directions are counterclockwise or
clockwise, the curvature signs are negative or positive, and the
bending angles are 90 degrees or 180 degrees. The sum total of the
products of the curvature signs and the bending angles of the
curved waveguides 412ba, 412bc, 412be, 412bg, 412bi, and 412bk is
450 degrees.
[0089] As a result, the sum total of the products of the curvature
signs and the bending angles of the curved waveguides 411b, 411c,
411e, 4111, 411m, 412ba, 412bc, 412be, 412bg, 412bi, and 412bk
(-90+450=360) and the sum total of the product of the curvature
sign and the bending angle of the ring waveguide 413b have the same
absolute value with opposite signs. Thereby, rotation of the
polarization plane of light generated in the ring waveguide 413b
and rotation of the polarization plane of light generated in the
curved waveguides 411b, 411c, 411e, 4111, 411m, 412ba, 412bc,
412be, 412bg, 412bi, and 412bk cancel each other out. As a result,
regardless of whether the input light L41 is in TE-polarization or
TM-polarization, light L43 is output while securing a high
polarization maintaining property.
[0090] In the optical waveguide circuit 400, the curvature radii of
each of the curved waveguides of the input connection waveguide 411
and the output connection waveguides 412a and 412b are different
from the curvature radii of the ring waveguides 413a and 413b so
that each of those can be set as an optimal value. Furthermore,
since the waveguide lengths of the ring waveguides 413a and 413b
are different from each other, it is possible to set different FSR.
For example, it is possible to design FSR of the ring waveguide
413a to be 75 GHz and design FSR of the ring waveguide 413b to be
50 GHz. Furthermore, by setting the curvature radii of each of the
curved waveguides of the input connection waveguide 411 and the
output connection waveguides 412a and 412b as 250 .mu.m, it is
possible to decrease light loss through the waveguides and to
reduce the size of the optical waveguide circuit 400. Furthermore,
with the optical waveguide circuit 400, the circulating directions
of light can be set to be opposite from each other for the ring
waveguides 413a and 413b, so that design thereof may be highly
flexible.
[0091] As described above, with the optical waveguide circuit 400,
it is possible to implement both high flexibility in designing FSR
and low bending loss while securing a high polarization maintaining
property. Note that the curvature radii and the bending angles
mentioned above are examples, and it is possible to set the values
thereof freely in accordance with the required bending loss and
FSR. Furthermore, in the optical waveguide circuit 400, one end of
the input connection waveguide 411 and one ends of the output
connection waveguides 412a and 412b are extended toward the same
side face. Therefore, it is easily connected to an optical fiber
array or another optical waveguide circuit.
[0092] As a fourth example, an optical waveguide circuit having the
configuration of the optical waveguide circuit 400 was manufactured
by the manufacturing method described above. The property of the
manufactured optical waveguide circuit was measured, and FSR of a
ring resonator configured with two ring waveguides was 75 GHz and
50.2 GHz. Furthermore, when TE-polarized light was input from an
input connection waveguide and the polarization mode coupling
amount was measured, a preferable value of -25 dB or less was
acquired. Moreover, when curved waveguide parts of the input
connection waveguide and the output connection waveguide were diced
and the light loss thereof was measured, acquired was a preferable
value of 0.95 dB/cm or less.
[0093] Note that the present disclosure is not limited by the
above-described embodiments. For example, in the third and fourth
embodiments, there may be three or more ring waveguides, two or
more branch parts, and three or more branched paths. Furthermore,
the present disclosure includes configurations acquired by
combining each of the above-described structural elements as
appropriate. Moreover, further effects and modifications may easily
occur to those skilled in the art. Therefore, broader aspects of
the present disclosure are not limited by the above-described
embodiments but various changes are possible.
[0094] According to an embodiment, it is possible to implement both
high flexibility in designing FSR of the ring resonator and low
bending loss while securing a high polarization maintaining
property.
[0095] Although the disclosure has been described with respect to
specific embodiments for a complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art that fairly fall within the
basic teaching herein set forth.
* * * * *